Metal coating process



United States Patent 3,449,222 METAL COATING PROCESS Dennis B. Freeman, Harrow, and Peter Burden, Burnham Beeches, Slough, England, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Aug. 12, 1965, Ser. No. 479,313

Claims priority, application Great Britain, Aug. 13, 1964,

33,074/ 64 Int. Cl. C23f 17/00 US. Cl. 204-38 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved method for coating metal surfaces and more particularly it relates to an improved process for providing a protective or paint base coating on a ferrous metal surface.

In forming protective coatings on ferrous metal surfaces, that is, on surfaces which are predominantly of iron, it is standard practice to treat the surfaces with an aqueous acidic zinc phosphate solution. Normally, this treatment with a zinc phosphate solution is carried out by immersing the surface in the solution or by spraying the solution onto the surface. Inasmuch as the formation of the coating depends to a large extent upon the acid attack on the metal, when a phosphate layer is formed on the metal, it largely inhibits further attack and the coating is then substantially complete. The time needed to effect the formation of such a complete coating will vary, with times up to about 1 minute being typical when the coating solution is applied by spraying and times up to about minutes being typical when the ferrous surface is immersed in the coating solution.

The coatings thus-produced, even when they are of a very fine grain size, are porous to some extent and, hence, may not afford complete corrosion protection. It is, therefore, customary to seal the coating by the application of an oil, a paint, or similar protective coating. Accordingly, such zinc phosphate coatings are used extensively as a. base on which a paint layer. is formed, although they do provide some corrosion resistance in themselves beneath the paint layer.

With the recent development of water-soluble resin base paints, a great deal of work has been done on the application of such paints by electrophoresis. The electrophoretic method of painting involves the phenomena of electroosmosis and electrolysis, as well as electrophoresis. In this method, an electric current is passed through the paint while the article to be painted is made an electrode, usually the anode, in the paint.

Conventional Zinc phosphate coatings, as have been described hereinabove, have not been found to be entirely satisfactory as a base on which paint is applied using these electrophoretic techniques. The principal disadvantage encountered is that these zinc phosphate coatings have a significant electrical resistance which, although in some instances may be advantageous in insuring a good distri bution of paint applied electrophoretically, has been found to limit the thickness of the paint deposits which can be produced. In many instances, the higher resistance and hence, the low conductivity of the zinc phosphate coating, has made it very difficult to apply electrophoretically, paint coatings, of the thickness which is required. Additionally, it has been found that when the paint is electrophoretically deposited on and in a conventional, crystalline zinc phosphate coating, there is an appreciable reduction in the gloss of the paint film.

It is known in the art that a zinc phosphate coating can be deposited by treating ferrous surfaces with a zinc phosphate solution while the under the influence of an electrical potential. For example, in US. Patent 1,856,261, it is taught that the formation of a dense, black, zinc phosphate coating may be fo med by the application of an electric current, after which the current is discontinued and the deposition of phosphate from the zinc phosphate solution is allowed to continue as in a normal immersion process. In this process, it is further taught that the electric current may be supplied intermittently during the process. In this process, however, the zinc which is electro-deposited on the surface is converted to zinc phosphate so that the coatings produced still have a high resistance. Additionally, such coating also are found to reduce the gloss of a subsequently applied paint. Moreover, it is found that in operating the process of this patent, the use of high phosphate concentrations, for example, 5 to 8% is necessary and the free acidity of the coating solution must also be controlled, for example within the range of about 0.05 to 0.13%. Accordingly, processes of this type have been found not to provide a satisfactory base for electrophoretically applied paint.

It is, therefore, an object of the present invention to provide an improved method of producing a zinc phosphate coating which has good corrosion resistance even without the application of paint or similar protective coatmg.

A further object of the present invention is to provide an improved method for producing zinc phosphate coatings on ferrous metal surfaces, which coatings do not adversely affect the gloss of a paint which is subsequently applied over the coating.

Another object of the present invention is to provide improved zinc phosphate coatings on ferrous metal surfaces which coatings have controlled electrical resistance.

A still further object of the present invention is to pro vide an improved zinc phosphate coating on ferrous metal surfaces.

Pursuant to the above objects, the present invention includes a process for treating a ferrous metal surface which comprises contacting the surface to be treated with an acidic zinc phosphate coating solution, maintaining the solution in contact with the surface for a period of time at least snfiicient to form a zinc phosphate coating on the surface and, thereafter, passing a direct electric current through the zinc phosphate coating solution, with the zinc phosphate coated surface as a cathode in the solution, so as to deposit Zinc on the zinc phosphate coated surface and continuing passage of the current until the zinc is deposited in at least the pores of the zinc phosphate coating on the surface. By the use of this method, it has been found that the metallic zinc deposited at least in the pores of the zinc phosphate coating gives a reduction in the electrical resistance of the coatings, thus making them more suitable as bases on which paint can be applied. The coatings obtained by the present process are found to be extremely smooth and the gloss of paint applied over these coatings is much higher then that of paint which is applied over conventional crystalline phosphate coatings. Additionally, the corrosion resistance of the coatings obtained is found to be greatly improved over that of conventional zinc phosphate coatings, both in the unpainted 3 state and when subsequently painted, electrophoretically or otherwise.

More specifically, in the practice of the present invention, a ferrous surface to be treated is contacted with an aqueous acidic zinc phosphate coating solution, which solution is maintained in contact with the ferrous surface for a period of at least sufficient to form a protective zinc phosphate coating on the surface. Various methods may be used for effecting the contact of the ferrous surfaces with the Zinc phosphate solution, such as spraying, immersion, flooding, and the like. Preferably, the contact is effected by immersing the surface in the phosphate solution, the workpiece to be coated being suspended in the solution by an electrically conducting support, so that once the formation of the zinc phosphate coating is substantially complete, it is only necessary to turn on the electric current to effect the deposition of metallic zinc on the surface. It is to be appreciated, however, that the application of the Zinc phosphate coating solution may also be carried out by spraying the coating solution on the surface, as by passing the workpiece to be coated on a conveyor to a conventional spraying Zone. Thereafter, the zinc phosphate coated workpiece may be carried by the conveyor into electrical contact with an electrical conductor through which the electrical current is passed while the workpiece is immersed in a body of the coating solution. Alternatively, after the workpiece has been coated with the Zinc phosphate coating by spraying, the work may be connected to an electrical supply, so that the work is cathodic, and the surface may be flooded with the zinc phosphate coating solution until the desired deposition of metallic zinc has taken place.

After the desired Zinc phosphate coating has been formed on the ferrous surface, the coated surface is contacted with an aqueous acidic zinc phosphate coating solution while an electrical current is passed through the solution, with the ferrous metal surface to be coated as the cathode. The passage of the direct electrical current is carried out so as to deposit zinc on the coated surface and is continued until the zinc is deposited in at least the pores of the zinc phosphate coating on the surface. Preferably, the chemical coating of the ferrous surface with the zinc phosphate coating solution and the deposition of zinc on the coated surface from the zinc phosphate coating solution is effected in the same volume or body of coating solution, since in this manner only a single control for the zinc phosphate coating solution is necessary. Alternatively, however, separate tanks or portions of the zinc phosphate coating solution may be used, which separate solutions are, preferably, of the same composition. It is to be appreciated, however, that, if desired, coating solutions having different compositions may be used in the chemical coating and the electrolytic steps. Generally, such a procedure is not preferred since, as with the use of separate volumes of coating solution, individual control of each of the coating solutions is necessary. Thus, in its most preferred embodiment, the ferrous surface to be coated is immersed in an aqueous acidic zinc phosphate coating solution for a period of time sufficient to effect the formation of the zinc phosphate coating on the surface and, thereafter, with the ferrous surface as the cathode in the solution, a direct electrical current is passed through the solution to effect the deposition of zinc on the phosphate coated surface.

Following the application of the zinc enriched zinc phosphate coating to the surface, the surface may be painted. The paint may be applied using either conventional means, or desirably, using electrophoresis. The electrophoretic application of the paint may be carried out in various ways, as are known to those in the art. Typically, the coating solutions utilized are dilute aqueous solutions, having a solids content within the range of about 3 to percent solids. As has been indicated, the metal to be coated is preferably the anode and the voltages used are typically within the range of about 50 to 1000 volts (direct current). Typical current densities used and coating times required are, respectively, from about 0.1 to 7 amperes per square foot and from about 10 seconds to about 2 minutes. Normally, the coating solution is at substantially room temperature, i. e., 30 to 40 degrees centigrade or even higher, may be used if desired. The paint applied using these techniques are Water-thinned resin paints which are, typically aqueous solutions based on synthetic resins such as alkyd resins, acrylic polymers, melamine resins, and the like. These aqueous resin solutions generally have a pH of about 9 and the solvent used is either water or an aqueous alcoholic mixture.

Various zinc phosphate solutions which are useful for forming protective coatings on ferrous surfaces, as are known in the art, may be used in the process of the present invention. Typically, these solutions are aqueous acidic solutions containing zinc ions and phosphate ions and may be formulated from zinc phosphate, zinc oxide and phosphoric acid, or other suit-able sources of zinc ions and phosphate ions. Additionally, these solutions may also contain an accelerator or oxidizing agent, such as nitrate ions, nitrite ions, chlorate ions, peroxide ions, and the like, as are well known in the art. Additionally, other modifying ions may also be present in the solutions, such as ferrous ions, nickel ions, alkaline earth metal ions, such as calcium ions, copper ions, and the like.

It has been found, that to some extent, the particular accelerator or oxidizing agent used may effect the nature of the coating formed on the surface. Thus, where the accelerator used comprises nitrate ions and the solution also contains ferrous ions, or where the accelerator is nitrite, with or without additional nitrate ions, dark gray coatings are obtained. Where, however, the accelerator is chlorate or peroxide, the coatings obtained are light gray in color and often have a higher metallic zinc content. In some instances, it has also been found that the bath operating conditions may be varied more when using a chlorate accelerator, and for this reason, this type of accelerator is preferred.

Exemplary of zinc phosphate solutions which may be used are those described in U.S. Patents 2,835,617; 3,090,- 709; 3,161,549; 2,813,812; 2,743,204; 3,015,594; 2,540,- 314; 2,514,149; 2,591,479; and 2,487,137. Although the specific concentration of the phosphate solutions used may depend on the type of coating desired and the specific operating conditions used in each instance, in general it has been found to be desirable that the solutions used contain from about 0.5 to about 3% by weight of phosphate (P0 and have a free phosphoric acid content within the range of about 0.3 to about 0.8%. Additionally, as is set forth in the various patents listed hereinabove, the accelerators and modifying ions are typically present in amounts within the range of about 0.0001 to about 5% by weight of the solution, depending upon the particular ions added and the nature of the coating which is desired.

As has been noted hereinabove, the zinc phosphate solution for both the zinc phosphate coating step and the electrolytic zinc depositing step may be applied using various coating techniques. Although the temperature of the zinc phosphate solution during the deposition of the zinc phosphate coating has not been found to be critical, temperatures from room temperature up to the boiling point of the solution, i.e., about 20 degrees centigrade to about 100 degrees centigrade, being satisfactory, it has been found that during the passage of the electric current, high temperatures may favor the deposition of phosphate with the metallic zinc, particularly where nitrate or nitrite accelerators are used. For this reason, it is desirable that the temperature of the zinc phosphate solution, at least during the passage of the direct current through the solution, is not in excess of about degrees centigrade and preferably is within the range of about 20 to about 70 degrees centigrade. Additionally,

it has been found that in some instances the use of extremely high current densities during the zinc deposition portion of the process may have an adverse aflFect on the coatings produced. Accordingly, it has been found to be generally desirable to use current densities which are not substantially higher than about 30 amperes per square foot, with current densities within the range of about to about amperes per square foot being preferred.

Although the duration of contact time between the coating solution and the surface to be treated will vary according to the nature of the coating desired and the solution operating conditions, contact times of from several seconds, e.g. 5, up to about 5 minutes are typical, depending on whether spray or immersion application techniques are used. Additionally, with regard to the electrolytic zinc deposition of the process, it has been found to be desirable if the direct electric current is applied to the zinc phosphate solution for a period of from about A of the time to a time equal to that with which the surface is in contact with the solution during the deposition of the zinc phosphate coating. Thus, in an immersion coating process, where the workpiece is contacted with the coating solution for about four minutes to produce the zinc phosphate coating, the electric current will be passed through the solution for a period of time of from about 1 to about 4 minutes to eflect deposition of zinc onto the zinc phosphate coating.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are given in degrees centigrade and percents are by weight.

Example 1 A concentrate was prepared by admixing 120 parts of zinc oxide, 375 parts of phosphoric acid, and sufficient water to make a total of 1000 parts. 100 parts of this concentrate was then diluted in water to a total volume of 1 liter. The resulting solution had a total acid point age of 50 and a free acid pointage of 8.0. This solution was then heated to about 65 degrees centigrade and 1 gram of sodium nitrite was added to the solution. Steel panels were then immersed in the solution for four minutes and, thereafter, a direct electric current was passed through the solution, using the panels as cathodes, for a period of 1 minute using a current density of 15 amperes per square foot. The resulting panels were designated as panels A. Similar steel panels were then coated by immersing them in a similar solution for a period of 5 minutes, but without any passage of electric current through the solution. These panels were designated as panels B. The corrosion resistance of panels from both sets, as produced hereinabove, were then tested by subjecting the panels to a standard salt spray test using sodium chloride solution at room temperature and the standard 100% humidity test at 50 degrees centigrade, both tests being carried out for a total of 96 hours, and the following results were obtained:

Salt spray, percent Humidity, percent The procedure of Example 1 was repeated with the exception that phosphating times of two minutes and one minute and zinc deposition times of 30 seconds and fifteen seconds respectively, were used. In each instance, similar corrosion resistant results were obtained.

Example 3 A coating solution was prepared as in Example 1 and steel panels, C1 were wiped clean with mineral spirits and immersed in the solution for 1 minute, the solution being at a temperature of about 70 degrees centigrade. Thereafter, a direct electric current at a current density of 15 amperes per square foot was passed through the solution for 15 seconds, with the panels as the cathode, after which time the panels were removed from the solution, rinsed in water and dried. Similar panels, C2. and C4 were similarly treated using immersion times of two minutes and four minutes, respectively and current passing times of /2 minute and one minute, respectively. Another set of panels, D, was immersed for 5 minutes in a commercial zinc phosphate solution containing about 0.65% Zinc, 1.34% P05, 0.5% N0 and 0.001% N0 This solution was at a temperature of about 75 degrees centrigrade. Some of the panels from C1, C2, C4 and D were then painted with a White alkyd stoving paint and panels from all of the sets, both painted and unpainted, were subjected to the salt spray and humidity test as in Example 1. In addition to the percent rusting determination made on the unpainted panels, the spread of underpaint corrosion and the blistering on the painted panels were also determined. The amount of blistering was measured in accordance with the American Society for Testing Materials procedure -D-714. Using the above procedure, the following results were obtained:

Painted panels Spread of underpaint corrosion (millimeters) Salt hot spray, humidity, 550 hrs. 820 hrs. 820 hrs.

9 1 Few 6. 7 1 Few 5, m

dense 8. 4 1 15 Unpainted panels, percent rusting Salt Hot spray, humidity, 168 hrs. 168 hrs.

Blistering in hot humidity,

D0. 2 Few 5, in dense 6.

The process of the preceding example was repeated with the exception that the processing solution used was a commercial zinc phosphate solution containing about 0.24% zinc, 1.02% P0 and about 0.35% chlorate. This solution had a total acid pointage of 20 and a free acid pointage of about 3.0. The steel panels were treated by immersing for 4 minutes in the solution, at a temperature of about 70 degrees centigrade, followed by the passage of a direct electric current at a current density of 10 amperes per square foot for 1 minute. These panels, painted with the white alkyd stoving paint as in the preceding example, were then subjected to the Erichsen window test and a value of 75 was obtained. Similar panels treated with the same phosphatizing solution but without the application of the electric current and commercial electro-galvanized panel-s, both of which were painted with the same white alkyd stoving paint, gave values of 55 and -65, respectively, when subjected to the same test. The Erichsen window test value is the number of 1 millimeter scribed squares of paint remaining after the application of a 10 millimeter Erichsen depression.

Example 5 Steel panels 5 inches by 2.8 inches were treated with the solutions using the procedures as described in the preceding example. Thereafter, a black water soluble paint, containing 12% by weight solids, was electrophoretically applied to the panels, the paint application being carried out for 2 minutes at a voltage of 60 volts; with the panel as the anode. The painted panels were then stored for 30 minutes at degrees centigrade. Similar panels were treated using the same solution but without the application of the direct electric current to deposit zinc and were electrophetically painted in the same manner. In each instance, the weight and thickness of the paint film obtained were recorded and the gloss of the paint film on the panels were observed. Using this procedure, the following results were obtained:

l Unprocessed (plain steel).

2 Panel 7 was phosphated for 1 minute without electrolysis, followed by 1 minute with electrolysis at 10 amps per square foot, followed by 1 minute without electrolysis. From the above results, it is quite apparent that the coating on the panels treated in accordance with the process of the present invention, namely, 4, 5 and 6, are heavier and thicker than the coatings produced on the panels 1, 2, 3 and 7. Additionally, it was noted that the paint gloss on panels 4, 5, and 6 was substantially the same on each and the paint gloss on panels 1, 2, 3 and 7 was substantially the same on each but that the paint gloss on panels 4, 5 and 6 was appreciably greater than the paint gloss on panels 1, 2, 3 and 7.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method for treating a ferrous metal surface which comprises contacting the surface to be treated with an acidic zinc phosphate coating solution, maintaining the solution in contact with the surface for a period at least sufficient to form a zinc phosphate containing coating on the surface by chemical action, and thereafter, passing a direct electric current through the zinc phosphate solution, with the zinc phosphate coated surface as a cathode in the solution, so as to deposit zinc on the surface from the solution and continuing passage of the current until the zinc is deposited in at least any pores of the zinc phopsphate coating.

2. The method as claimed in claim 1 wherein the zinc phosphate coating solution contains from about 0.5 to about 3% of phosphate, measured as P0 and a free phosphoric acid content of from about 0.3 to about 0.8%.

3. The method as claimed in claim 2 wherein the zinc phosphate coating solution also contains an accelerator.

4. The method as claimed in claim 3 wherein the zinc phosphate coating solution is at a temperature within the range of about 20 to about 70 degrees centigrade and the zinc is deposited on the zinc phosphate coating by passing a direct electric current through the solution at a current density which is not in excess of about 30 amps per square foot.

5. The method as claimed in claim 4 wherein the ferrous surface to be treated is immersed in the zinc phosphate coating solution for a period of from about 1 to about 5 minutes and the direct electric current is passed through the solution for aperiod of time of from about A to a time equal to that of that which the surface is immersed in the solution.

6. A ferrous metal article having a surface produced in accordance with the procedure of claim 1.

7. A process for treating a ferrous metal surface which comprises chemically depositing a zinc phosphate containing coating on the surface from an acidic zinc phosphate coating solution and, thereafter electrolytically, depositing metallic zinc 0n the zinc phosphate coated surface, the amount of zinc deposited being at least sufficient to fill any pores in the zinc phosphate coating.

References Cited UNITED STATES PATENTS 1,856,261 5/1932 Phillips et al. 204 2,056,399 10/ 1936 Hochwalt et al. 20438 2,412,543 12/ 1946 Tanner 20438 1,007,069 10/1911 Coslett 20456 2,125,387 8/1938 Mason 20456 2,132,438 12/1933 ROmig 20438 JOHN H. MACK, Primary Examiner.

W. B. VAN SISE, Assistant Examiner.

US. Cl. X.R. 

